THE NOBEL PRIZE CENTENARY IN PHYSICS by fuf15836

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									                    THE NOBEL PRIZE CENTENARY IN PHYSICS
                          A.N. Dovbnya, V.A. Shendrik, A.A. Shendrik1
                          Scientific-Production Complex “Accelerator”
             National Science Center “Kharkov Institute of Physics and Technology”
                        1, Akademicheskaya St., 61108 Kharkov, Ukraine
                      1
                        Secondary School, Valki, Kharkov Region, Ukraine
The report is dedicated to the discovery of X-rays and their revolutionary effect on the formation and development
of modern physics.
PACS numbers: 01.90.+g, 01.65.+g

    On the 10th of 1901, in the Large Hall of the Acad-      while the bones were nearly opaque to it.
emy of Music in Stockholm the Nobel Prize Committee              A unique talent of physicist-experimenter, excep-
awarded Roentgen with the first Nobel Prize in physics       tional powers of observation, and a firm rule to attain
as a mark of gratitude of scientists and the mankind.        clarity in everything permitted Roentgen to discover the
    Wilhelm Konrad Roentgen was born in a small              phenomenon which had been for many years close by
German town of Lennepe not far off from the Germany-         the scientists who made experiments using the same
Netherlands frontier. He devoted all his life to physics.    devices.
Having become Professor of Physics, Roentgen gave                However, the character of this new radiation re-
lectures on physics at a number of institutes in Ger-        mained enigmatic. Only one thing was clear, i.e., the
many.                                                        radiation could not be identified with the cathode rays.
    The physical experiment was in his element. Hardly       Similarly to the cathode rays, it gave rise to fluores-
anybody could be compared with him in thinking out           cence, had a chemical action, propagated in straight
the experiment, in the accuracy of measurements and in       lines, formed shadows. However, the X-rays did not
thoroughness of analyzing possible mistakes. Roentgen        have the characteristic properties of the cathode rays -
had already become famous among physicists of that           they were not deflected by the magnetic field. Maybe
time for his investigations in various areas. Thus, for      they were of the same nature as the ultraviolet radiation
example, in 1890, he was the first to prove by direct        was? But in that case they should be appreciably re-
experiment that moving charges generate a magnetic           flected, refracted, polarized.
field. At the end of the 19th century, W. Roentgen,              Those were the questions (repeating an attempt to
Professor of the Wurzburg University in Germany,             explain the nature of the rays), with which Roentgen
conducted experiments with electric discharge in gases.      finished his first work on X-rays, reported at the Physics
He used a glass tube having two electrodes soldered in       Institute of the Wurzburg University in December,
it and pumped down to a pressure of about 10-5 of at-        1895.
mospheric pressure. When a high voltage was applied              The first article of the scientist “About a new kind of
to the electrodes, the glass about the anode started         rays”, where he described the properties of the radiation
glowing with a yellow-green light. This glow was at-         discovered by him, aroused an enormous interest
tributed by physicists to the action of the so-called        throughout the world and was then published as a sepa-
cathode rays, the flux of which was emitted by the           rate brochure in all European languages.
cathode, and was incident on the anode and, partially,           The second work reported on 5 March, 1896, com-
on the tube walls.                                           prised two new essential facts. The first was that under
    One November evening of 1895, when working in            the action of X-rays the electrified bodies get dis-
the laboratory, Roentgen hit upon an unusual phenome-        charged. It is not the X-rays themselves but the air
non. For experiments, he wrapped the discharged tube         penetrated by them that acquires the property to dis-
with a black light-proof paper. It was dark in the room,     charge the electrified bodies. The second important fact
and this allowed the scientist to notice that the barium     mentioned even in the first Roentgen′s work was that X-
salt crystals lying not far from the tube radiated a faint   rays were produced with the cathode rays hitting not
light. He deenergized the tube and the luminescence          only the glass of discharge tubes, but also any sub-
disappeared. Then Roentgen placed a barium salt-coated       stance, not excluding liquids and gases. Depending on
screen not far from the tube, and the screen started         the character of substance struck by the cathode rays,
glowing. The scientist began placing different objects       the intensity of the resulting X-radiation turned out to be
between the tube and the screen. Cardboard, paper, eb-       different. Those observations brought Roentgen as early
onite plates exerted no effect on the brightness of the      as in February, 1896, to the development of the “focus”
glow. Metal subjects cast a shadow on the screen. Evi-       tube, where a concave aluminum mirror served as a
dently, the tube was a source of unknown penetrating         cathode and a platinum plate placed at the centre of cur-
rays, X rays, as Roentgen called them, or Roentgen rays      vature of the mirror and inclined at 45° to the mirror
as we now call them.                                         axis served as an anode. Before the advent of thermionic
    The researcher put his hand in the path of X-rays,       devices, the “focus tubes were the only setups to pro-
and a dark image of the hand skeleton appeared on the        duce X-rays for medical and physical investigations.
screen - soft tissues were transparent to the radiation,     Roentgen did much to quickly promote his discovery,
   ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №3.                                                                        33
 Серия: Ядерно-физические исследования (38), с. 33-34.
having rejected with his characteristic disinterestedness      the values calculated in other ways. Of all the methods
any possibility of making a profit from it. The general        of determining ħ the method based on the measurement
interest much contributed to a rapid progress of X-ray         of the sort wavelength boundary of the continuous X-
engineering. It will suffice to give only one example to       ray spectrum is believed to be most exact.
illustrate the path covered: in 1896 the radiography of a          At a rather high electron velocity, apart from the
hand took a 20 min exposure, while now an instant is           continuous X-ray radiation (i.e., the radiation due to
sufficient for the purpose.                                    electron deceleration), the characteristic radiation is also
    The physicists who held the viewpoint that X-rays          excited (generated by excitation of inner electron shells
were the electromagnetic radiation naturally tried to          of anticathode atoms). While the continuous X-radiation
detect not the reflection but the diffraction on extremely     is independent of the anticathode material and is deter-
narrow slits, as dictated by the supposed small value of       mined only by the energy of electrons bombarding the
X-ray wavelength. However, the man-made slits, no              anticathode, the characteristic radiation is specified by
matter how narrow they were, appeared to be too rough.         the nature of substance, from which the anticathode is
Besides, it was clear that it was difficult, if possible, to   made. As long as the electron energy is insufficient to
find the mechanical way of scribing rulings being well         excite the characteristic radiation, only continuous X-
off at a distance of about a molecular size. In 1912, the      ray radiation arises. At a sufficiently high energy of
German physicist M. Laue put forward a bold idea to            bombarding electrons, sharp lines of the characteristic
use crystals as diffraction gratings for X-rays. In the        spectrum appear against the background of the continu-
same year, the theory was corroborated by experiments.         ous X-ray spectrum, the intensity of these lines being
In 1914, for the discovery of X-ray diffraction by crys-       many times higher than that of the background.
tals M. Laue was awarded the Nobel Prize in physics.               The characteristic X-rays were discovered in 1906
    And yet, Roentgen could not explain the nature of          by the English physicist Ch. Barkla, and in 1917 he was
enigmatic rays. He did not know about the existence of         also awarded with the Nobel prize in physics. In 1913,
electrons, and it was their slowing down in the tube           another English physicist H. Moseley established a sim-
glass that was the reason for the appearance of X-rays         ple law relating the frequency of spectral lines from the
and a greenish visible light. When a charged particle          characteristic X-ray radiation to the ordinal number of
comes flying into the substance, it slows down, loses its      the emitting element (Moseley law). The dependence
velocity and emits electromagnetic waves. The X-               established by Moseley allows one to determine exactly
radiation wavelengths range from 5⋅10-8 to 5⋅10-12 m. On       the atomic number of the given element from the meas-
the scale of electromagnetic waves they take the place         ured wavelength of X-ray lines; it has played a great
between ultraviolet radiation and gamma-radiation. The         role in the arrangement of elements in the periodic sys-
beam of slowing down electrons emits waves of a wide           tem.
diversity of wavelengths. These waves form a continu-              W. Roentgen devoted his life to classical physics.
ous X-ray spectrum. The wavelength which accounts for          But it was his discovery of “a new type of rays” that
the maximum intensity of radiation should decrease as          was the starting point for the development of new
the electron velocity increases, i.e., as the tube voltage     physics - physics of atom and atomic nucleus. In less
increases. Experiments have established the short              than half a year after the discovery of X-rays, in an at-
wavelength boundary of the continuous X-ray spectrum           tempt to puzzle out their nature the radioactivity was
λmin = 12390/U, where λmin is expressed in angstroms,          discovered, and a year later the electron was found with
and U - in volts.                                              their use.
    The existence of the short wavelength boundary di-             The discovery of X-rays had extremely important
rectly follows from the quantum nature of the radiation.       consequences for both scientific investigations and
Really, if the radiation arises at the expense of energy       practical applications in medicine and industry. It will
lost by the electron in its slowing down, then the quan-       not be an exaggeration to say that from this discovery
tum value ħω cannot exceed the electron energy eU:             new present-day physics begins.
ħω ≤ eU. Hence it turns out that the radiation frequency
cannot exceed ωmax = eU/ħ, and therefore, the wave-
length cannot be smaller than λmin = 2πc/ωmax =
= 2πħc/eU. Thus we have arrived at the empirical rela-
tion given above. The ħ value found from these rela-
tions for the Planck constant is in good agreement with




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